making strong colours in a rainbow cake

I have a wedding cake to do where the bride wants a rainbow cake style. She showed me a pic where all the colours are in stripes.

My question is, though I know how to do this sort of cake, how do I ensure that the colours come out nice and bright and also what about making sure the correct colour comes out!

My batter is usually a soft pale yellow just before I bake it. So my thoughts are that when I add say blue colouring, it might turn a shade of green because of the yellow colour to start with. Same with any other colour that may change when added to a pale yellow colour, like red going bit orangy etc.

I would start with a white cake batter. If you want to insure that the colors come out as you want I would do a trial run with cup cakes, the size in batter is different but at least you can see if the color is in the family that you want, ie red not orange, blue not green.

thanks for replying. How do you make a white cake batter? I'm in the UK (whether that makes a difference or not)?! and usually make all my cakes from scratch. Seeing as the butter is a yellow or pale yellow that I use, unless I beat it to death, its gonna stay yellowish!! Is white cake a box mix or can you do it from scratch?

There are recipes on here for white cakes, although some of the US ones will refer to box mixes. Also it is the use of egg whites only rather than whole eggs! I'm not sure about the fat/marg, maybe use oil instead?

The AmeriColor brand has some "Electric" colors that show up incredibly bright in my rainbow cakes.

I usually use AmeriColor's Electric Blue, Electric Green, Electric Pink, Electric Yellow, and Electric Purple. They are sold in large and small bottles. It only takes a few drops to get a bright color, so the small bottles should be more than sufficient. They are fairly inexpensive at around $1.50 for each color. I have used these in a yellow cake and all of the colors stayed pretty true to what was expected, with the exception of the blue. The blue was still blue, but had a slight green tint to it. It wasn't a very noticeable tint.

I have used any white box cake mix..I like DH...and added the Wilton dyes to the batter. This is the only additive that I have found to give you that true, vibrant color. Of course, "doctor' up your box mixes. You can find the Wilton colors in any Michael's, AC Moore, Joanne's or any craft store..Wal-mart even sells the cheap. Hope this helps!

White cakes as just Egg whites used not Egg Yolks- Check out recipes online. Also Use clear Vanilla if you are able Wilton makes a good one. This will insure the cake is white. Also Bleached flour if you aren't using cake flour and just all purpose kind. That is the way to ensure you have a fully white cake.